Tuesday, May 4, 2010

Prism Duty

Yesterday I put my education to good use carrying a mirror around in a big field. An assistant surveyor used to be called a "rod man," but nobody uses rods any more. Instead you carry a fancy mirror called a "prism." We were laying out a square grid across the field. So one person (me) paces out into the field to approximately the right place and points the mirror at the transit. The other guy stands at the transit and pushes a button that shoots a laser beam at the prism and measures its position to the tenth of a millimeter. He then calls out "north 15 centimeters, west 7 centimeters" or whatever, and the assistant moves the prism until it is in the right place. The only tricky part is keeping the prism straight upright; there is a level on the side, but it is a very sensitive level indeed. The best part was that the guy by the transit was a Brit, and every time I found the right spot he said, "Brilliant!"

The point of this exercise was so the Brit, Tim Horsley, could carry out a survey of this field using a high-resolution magnetometer. That thing around his neck doesn't look like much, but it costs about \$20,000 and measures extremely small changes in the local magnetic field. Two of the things that produce changes in the local magnetic field are those old human standbys, building fires and digging holes. So an archaeological site usually produces a lot of magnetic noise that this machine can pick up and map. Underneath the grass of this field are, we think, at least two Indian villages dating to AD 1400 to 1550. And although we didn't find any colonial artifacts here, there is an old private collection said to come from here that includes early colonial stuff. We are hoping that this high-tech approach will allow us to unravel what is under ground here without our having to dig it all up. I'll let you know how it turns out.